Like all British K-class submarines, K9 had a displacement of 1800LT when at the surface and 2600LT while submerged.[1] It had a total length of 338feet, a beam of 26feet, and a draught of 20feet. The submarine was powered by two oil-fired Yarrow Shipbuilders boilers and one geared Brown-Curtis or Parsons steam turbine; this developed 10,500 ship horsepower (7,800 kW) to drive two 7inchesft6inchesin (ftin) screws. It also contained four electric motors each producing 350to. It was also fitted with a diesel engine providing 800hp to be used when steam was being raised, or instead of raising steam.[2]
The submarine had a maximum surface speed of 24kn and a submerged speed of 9to.[3] It could operate at depths of 150feet at 2kn for 80nmi.[1] K9 was fitted with a 3inches anti-aircraft gun, ten 18inches torpedo tubes, and two 4inches deck guns. Its torpedo tubes were fitted to the bows, the midship section, and two were mounted on the deck.[1] Its complement was fifty-nine crew members.[3]